This invention relates to a white balance circuit arrangement for a color TV camera and, more particularly, to a circuit arrangement holding a white balance in a color TV camera having a solid-state image sensor.
Since a plurality of primary color signals are combined to form a picture in a color TV camera, it requires a white balance circuit to adjust a gain of each primary color signal so as to form a picture with proper coloring.
FIG. 1 shows a block diagram of a conventional white balance circuit. In the circuit an image sensor 1 outputs signal R, signal B, and signal Y depending on the incident light. The signals R and B, which are alternatively outputted as a line sequential signal, indicate primary colors and the signal Y indicates luminance. These signals are amplified through a preamplifier 2 and fed into a process amplifier 3 where color difference signals R-Y and B-Y are generated. An encoder 4 encodes the color difference signals so as to provide an output terminal 5 with a color video signal applicable, for example, to the NTSC system.
The signals R-Y and B-Y are also fed into a balance control circuit 6 where gain control signals V.sub.R and V.sub.B are generated. The process amplifier 3 amplifies inputted signals R, B and Y based on values of the gain control signals V.sub.R and V.sub.B. Thus the circuit 6 can hold a white balance by properly controlling values of signals V.sub.R and V.sub.B.
An initial setting of the balance control circuit 6 is made at each time when a white balance switch (not shown) is momentarily turned on. In other words, in order to make the initial setting, a cameraman usually directs the camera to a white object and turns on manually the white balance switch, thus setting a balanced condition, and thereafter, the gain of the process amplifier 3 is fixed to keep the balanced condition. However, since the gain condition set by the initial setting is fixed until the next time the white balance switch in manipulated, a desirable white balance is not always assured for objects being viewed under ambient lighting having different color temperatures between manipulations of the white balance switch. On the other hand, in an automatic white balance system where a white balance operation is always active for taking different color temperature scans, the image obtained tends to become whitened and looks unnatural when the image sensor is directed to a mono-colored non-white object. That is a drawback of the circuit shown in FIG. 1.
FIG. 2 shows a block diagram of another conventional white balance circuit. The circuit is provided with a color temperature sensor 7 which monitors the objects continually and gives a color temperature signal CT to a balance control circuit 8. The circuit 8 controls values of signals V.sub.R and V.sub.B based on the color temperature signal CT, thus holding a white balance continually.
In the circuit illustrated in FIG. 2, there are drawbacks such that the device becomes bulky because it is equipped with the color temperature sensor 7, and that it needs a time consuming gain adjustment to compensate the offset voltage of the diode in the sensor 7.